The predominant sandfly in Las Maria de Pavana, Choluteca, Honduras, was found to be Lutzomyia longipalpis, most (69%) of the 791 specimens of this species caught being male. When local Lu. longipalpis were studied over 1 year (1986-1987) using CDC traps, peaks in the size of the adult population were observed in December and July, each after a period of rain. Most [51% (24/47)-67% (97/144)] of the flies caught inside houses were female whereas most [55% (6/11)-56% (37/66)] of those caught outside were male. Far more Lu. longipalpis of both sexes were collected, per h of collection, from cattle and horses than from dogs or pigs, the vast majority (83%-93%) of the flies caught on each type of animal being male. The males may benefit from resting on the mammals because the females with which they mate come to the same animals for blood. The females may benefit by the presence of the males, not only by the increased chance of finding a mate but also because pheremones from the males may attract the females both to the males and to a bloodmeal source. The adult Lu. longipalpis only appeared to be active during the hours of darkness, none alighting in the twilight of dawn (04.00-06.00 hours) or dusk (18.00-20.00 hours) on the large mammals investigated.